ActivityPub

ActivityPub is a W3C Recommendation that defines a decentralized social networking protocol built from two complementary APIs: a client-to-server API for creating and reading content, and a server-to-server API for federating that content between independent instances. It is the protocol that powers the federated social web — the “fediverse” — including platforms such as Mastodon, PeerTube, and Pixelfed. Where centralized social platforms lock users into a single company’s API, ActivityPub lets many servers interoperate as one network.

• Two-sided API model - A client-to-server side for apps to publish and consume, and a server-to-server side for federation between instances.
• Built on ActivityStreams - Messages are ActivityStreams 2.0 objects, giving federation a shared, JSON-based vocabulary.
• Actor-based addressing - Every user, group, or service is an actor with an inbox and outbox that drive message delivery.
• Decentralized by design - No central authority; independently operated servers agree only on the protocol.

In practice, ActivityPub is one of the clearest working examples of federated API design at scale, which is why I keep coming back to it. Each server exposes and consumes the same standardized endpoints, so a post created on one instance propagates across thousands of others without any central coordinator — a model that has real lessons for enterprise API governance and interoperability. As organizations and even governments look at decentralized alternatives to platform lock-in, ActivityPub is the reference protocol for how independent APIs can federate into a coherent whole.